In high frequency technology power detectors are used, amongst other things, for generating the necessary feedback signal for power regulation. This signal is usually a sample taken with a directional coupler from a forward RF field and is rectified to a corresponding DC level that is dependent on the magnitude of the forward power.
The prior art is discussed in the following with reference to the accompanying FIG. 1, which presents a wiring diagram of a power detector according to the prior art.
In the circuit shown in FIG. 1, the directional coupler marked with the reference number 1 has been implemented utilising microstrip. When the sampled power is rectified, this yields the detected DC voltage level V.sub.det corresponding to the output power P.sub.out. In the FIGURE the power detector diode is marked with the number 2, the capacitors with the numbers 3, 4 and the resistors with the numbers 5, 6.
It is often necessary to make the directional coupler's coupling coefficient large so that with small voltage levels the DC voltage can be detected and its gradient of change can be made sufficiently large. This has the drawback that with large voltage levels the detected DC voltage level grows far too large. In devices operating at low operating voltage levels such as radio telephones, the processing of this signal, of which the level is excessively large, becomes impossible and thus the power range that must be treed is limited as a consequence of this.
A patent related to an automatic control circuit of the output of RF power amplifiers having a wide dynamic range has previously been presented (U.S. Pat. No. 4,602,218). As far as extending the power range is concerned, the circuit according to the patent is based on the use of a logarithmic amplifier. In the circuit according to the patent, a linearly dependent RF sample level (the coupling coefficient being constant) is taken, for the detector, from the power level with a coupling capacitor and then the sample is shaped with the logarithmic amplifier before the detection. This is a complicated way of implementing rite function in question and the solution calls for a lot of components.